Method and apparatus for processing video signal

ABSTRACT

A method for decoding a video according to the present invention may comprise: determining an intra prediction mode of a current block, deriving reference samples from neighboring samples of the current block, obtaining a first prediction sample for the current block, based on the intra prediction mode and the reference samples, determining an offset for the first prediction sample, and obtaining a second prediction sample by applying the offset to the first prediction sample.

CROSS REFERENCE TO RELATED APPLICATIONS

This application is a Divisional of U.S. patent application Ser. No.16/339,353 (filed on Apr. 3, 2019), which is a National Stage PatentApplication of PCT International Patent Application No.PCT/KR2017/011055 (filed on Sep. 29, 2017) under 35 U.S.C. § 371, whichclaims priority to Korean Patent Application Nos. 10-2016-0127862 (filedon Oct. 4, 2016) and 10-2016-0127863 (filed on Oct. 4, 2016), theteachings of which are incorporated herein in their entireties byreference.

TECHNICAL FIELD

The present invention relates to a method and an apparatus forprocessing video signal.

BACKGROUND ART

Recently, demands for high-resolution and high-quality images such ashigh definition (HD) images and ultra-high definition (UHD) images haveincreased in various application fields. However, higher resolution andquality image data has increasing amounts of data in comparison withconventional image data. Therefore, when transmitting image data byusing a medium such as conventional wired and wireless broadbandnetworks, or when storing image data by using a conventional storagemedium, costs of transmitting and storing increase. In order to solvethese problems occurring with an increase in resolution and quality ofimage data, high-efficiency image encoding/decoding techniques may beutilized.

Image compression technology includes various techniques, including: aninter-prediction technique of predicting a pixel value included in acurrent picture from a previous or subsequent picture of the currentpicture; an intra-prediction technique of predicting a pixel valueincluded in a current picture by using pixel information in the currentpicture; an entropy encoding technique of assigning a short code to avalue with a high appearance frequency and assigning a long code to avalue with a low appearance frequency; etc. Image data may beeffectively compressed by using such image compression technology, andmay be transmitted or stored.

In the meantime, with demands for high-resolution images, demands forstereographic image content, which is a new image service, have alsoincreased. A video compression technique for effectively providingstereographic image content with high resolution and ultra-highresolution is being discussed.

DISCLOSURE Technical Problem

An object of the present invention is to provide a method and anapparatus for efficiently performing intra-prediction for anencoding/decoding target block in encoding/decoding a video signal.

An object of the present invention is to provide a method and anapparatus for performing intra prediction through a weighted predictionusing a plurality of reference samples in encoding/decoding a videosignal.

An object of the present invention is to provide a method and anapparatus for refining a prediction sample generated through intraprediction using an offset in encoding/decoding a video signal.

An object of the present invention is to provide a method and anapparatus for refining a prediction sample generated through intraprediction using an offset, and further using a different offset in apredetermined unit in encoding/decoding a video signal.

The technical objects to be achieved by the present invention are notlimited to the above-mentioned technical problems. And, other technicalproblems that are not mentioned will be apparently understood to thoseskilled in the art from the following description.

Technical Solution

A method and an apparatus for decoding a video signal according to thepresent invention may determine an intra prediction mode of a currentblock, derive reference samples from neighboring samples of the currentblock, obtain a first prediction sample for the current block, based onthe intra prediction mode and the reference samples, determine an offsetfor the first prediction sample, and obtain a second prediction sampleby applying the offset to the first prediction sample.

A method and an apparatus for encoding a video signal according to thepresent invention may determine an intra prediction mode of a currentblock, derive reference samples from neighboring samples of the currentblock, obtain a first prediction sample for the current block, based onthe intra prediction mode and the reference samples, determine an offsetfor the first prediction sample, and obtain a second prediction sampleby applying the offset to the first prediction sample.

In a method and an apparatus for encoding/decoding a video signalaccording to the present invention, whether to apply the offset to thefirst prediction sample may be determined based on the intra predictionmode of the current block.

In a method and an apparatus for encoding/decoding a video signalaccording to the present invention, the offset may be determined basedon a weighted sum of the reference samples.

In a method and an apparatus for encoding/decoding a video signalaccording to the present invention, a weight applied to each of thereference samples is determined based on a distance from the firstprediction sample.

In a method and an apparatus for encoding/decoding a video signalaccording to the present invention, the reference samples include areference sample at a fixed position and a reference sample determineddependently on a position of the first prediction samples.

In a method and an apparatus for encoding/decoding a video signalaccording to the present invention, the reference sample at the fixedposition may include a reference sample adjacent to a top left corner ofthe current block, and the reference sample determined dependently onthe position of the first prediction sample may include at least one ofa reference sample on a same horizontal line as the first predictionsample or a reference sample on a same vertical line as the firstprediction sample.

In a method and an apparatus for encoding/decoding a video signalaccording to the present invention, each of the reference samples may beincluded in a different reference line.

The features briefly summarized above for the present invention are onlyillustrative aspects of the detailed description of the invention thatfollows, but do not limit the scope of the invention.

Advantageous Effects

According to the present invention, intra-prediction may be performedefficiently for an encoding/decoding target block.

According to the present invention, intra prediction can be performedbased on a weighted prediction using a plurality of reference samples.

According to the present invention, encoding/decoding efficiency may beimproved by refining a prediction sample generated through intraprediction.

The effects obtainable by the present invention are not limited to theabove-mentioned effects, and other effects not mentioned can be clearlyunderstood by those skilled in the art from the description below.

DESCRIPTION OF DRAWINGS

FIG. 1 is a block diagram illustrating a device for encoding a videoaccording to an embodiment of the present invention.

FIG. 2 is a block diagram illustrating a device for decoding a videoaccording to an embodiment of the present invention.

FIG. 3 is a diagram illustrating an example of hierarchicallypartitioning a coding block based on a tree structure according to anembodiment of the present invention.

FIG. 4 is a diagram illustrating a partition type in which binarytree-based partitioning is allowed according to an embodiment of thepresent invention.

FIGS. 5A and 5B are diagrams illustrating an example in which only abinary tree-based partition of a pre-determined type is allowedaccording to an embodiment of the present invention.

FIG. 6 is a diagram for explaining an example in which informationrelated to the allowable number of binary tree partitioning isencoded/decoded, according to an embodiment to which the presentinvention is applied.

FIG. 7 is a diagram illustrating a partition mode applicable to a codingblock according to an embodiment of the present invention.

FIG. 8 is a diagram illustrating types of pre-defined intra predictionmodes for a device for encoding/decoding a video according to anembodiment of the present invention.

FIG. 9 is a diagram illustrating a kind of extended intra predictionmodes according to an embodiment of the present invention.

FIG. 10 is a flowchart briefly illustrating an intra prediction methodaccording to an embodiment of the present invention.

FIG. 11 is a diagram illustrating a method of correcting a predictionsample of a current block based on differential information ofneighboring samples according to an embodiment of the present invention.

FIGS. 12 and 13 are diagrams illustrating a method of correcting aprediction sample based on a predetermined correction filter accordingto an embodiment of the present invention.

FIG. 14 shows a range of reference samples for intra predictionaccording to an embodiment to which the present invention is applied.

FIG. 15 is a diagram exemplifying a plurality of reference sample lines.

FIG. 16 is a flowchart illustrating a method for refining a predictionsample according to an embodiment of the present invention.

FIG. 17 is a flowchart illustrating a method for refining a predictionimage in a unit of a sub-block according to an embodiment of the presentinvention.

FIGS. 18 to 22 are diagrams illustrating intra prediction patterns of acurrent block according to an embodiment to which the present inventionis applied.

FIGS. 23 and 24 are diagrams illustrating examples of applying adifferent offset in a predetermined unit in a sub-block.

FIG. 25 is a flowchart illustrating processes of obtaining a residualsample according to an embodiment to which the present invention isapplied.

MODE FOR INVENTION

A variety of modifications may be made to the present invention andthere are various embodiments of the present invention, examples ofwhich will now be provided with reference to drawings and described indetail. However, the present invention is not limited thereto, and theexemplary embodiments can be construed as including all modifications,equivalents, or substitutes in a technical concept and a technical scopeof the present invention. The similar reference numerals refer to thesimilar element in described the drawings.

Terms used in the specification, ‘first’, ‘second’, etc. can be used todescribe various components, but the components are not to be construedas being limited to the terms. The terms are only used to differentiateone component from other components. For example, the ‘first’ componentmay be named the ‘second’ component without departing from the scope ofthe present invention, and the ‘second’ component may also be similarlynamed the ‘first’ component. The term ‘and/or’ includes a combination ofa plurality of items or any one of a plurality of terms.

It will be understood that when an element is simply referred to asbeing ‘connected to’ or ‘coupled to’ another element without being‘directly connected to’ or ‘directly coupled to’ another element in thepresent description, it may be ‘directly connected to’ or ‘directlycoupled to’ another element or be connected to or coupled to anotherelement, having the other element intervening therebetween. In contrast,it should be understood that when an element is referred to as being“directly coupled” or “directly connected” to another element, there areno intervening elements present.

The terms used in the present specification are merely used to describeparticular embodiments, and are not intended to limit the presentinvention. An expression used in the singular encompasses the expressionof the plural, unless it has a clearly different meaning in the context.In the present specification, it is to be understood that terms such as“including”, “having”, etc. are intended to indicate the existence ofthe features, numbers, steps, actions, elements, parts, or combinationsthereof disclosed in the specification, and are not intended to precludethe possibility that one or more other features, numbers, steps,actions, elements, parts, or combinations thereof may exist or may beadded.

Hereinafter, preferred embodiments of the present invention will bedescribed in detail with reference to the accompanying drawings.Hereinafter, the same constituent elements in the drawings are denotedby the same reference numerals, and a repeated description of the sameelements will be omitted.

FIG. 1 is a block diagram illustrating a device for encoding a videoaccording to an embodiment of the present invention.

Referring to FIG. 1 , the device 100 for encoding a video may include: apicture partitioning module 110, prediction modules 120 and 125, atransform module 130, a quantization module 135, a rearrangement module160, an entropy encoding module 165, an inverse quantization module 140,an inverse transform module 145, a filter module 150, and a memory 155.

The constitutional parts shown in FIG. 1 are independently shown so asto represent characteristic functions different from each other in thedevice for encoding a video. Thus, it does not mean that eachconstitutional part is constituted in a constitutional unit of separatedhardware or software. In other words, each constitutional part includeseach of enumerated constitutional parts for convenience. Thus, at leasttwo constitutional parts of each constitutional part may be combined toform one constitutional part or one constitutional part may be dividedinto a plurality of constitutional parts to perform each function. Theembodiment where each constitutional part is combined and the embodimentwhere one constitutional part is divided are also included in the scopeof the present invention, if not departing from the essence of thepresent invention.

Also, some of constituents may not be indispensable constituentsperforming essential functions of the present invention but be selectiveconstituents improving only performance thereof. The present inventionmay be implemented by including only the indispensable constitutionalparts for implementing the essence of the present invention except theconstituents used in improving performance. The structure including onlythe indispensable constituents except the selective constituents used inimproving only performance is also included in the scope of the presentinvention.

The picture partitioning module 110 may partition an input picture intoone or more processing units. Here, the processing unit may be aprediction unit (PU), a transform unit (TU), or a coding unit (CU). Thepicture partitioning module 110 may partition one picture intocombinations of multiple coding units, prediction units, and transformunits, and may encode a picture by selecting one combination of codingunits, prediction units, and transform units with a predeterminedcriterion (e.g., cost function).

For example, one picture may be partitioned into multiple coding units.A recursive tree structure, such as a quad tree structure, may be usedto partition a picture into coding units. A coding unit which ispartitioned into other coding units with one picture or a largest codingunit as a root may be partitioned with child nodes corresponding to thenumber of partitioned coding units. A coding unit which is no longerpartitioned by a predetermined limitation serves as a leaf node. Thatis, when it is assumed that only square partitioning is possible for onecoding unit, one coding unit may be partitioned into four other codingunits at most.

Hereinafter, in the embodiment of the present invention, the coding unitmay mean a unit performing encoding, or a unit performing decoding.

A prediction unit may be one of partitions partitioned into a square ora rectangular shape having the same size in a single coding unit, or aprediction unit may be one of partitions partitioned so as to have adifferent shape/size in a single coding unit.

When a prediction unit subjected to intra prediction is generated basedon a coding unit and the coding unit is not the smallest coding unit,intra prediction may be performed without partitioning the coding unitinto multiple prediction units N×N.

The prediction modules 120 and 125 may include an inter predictionmodule 120 performing inter prediction and an intra prediction module125 performing intra prediction. Whether to perform inter prediction orintra prediction for the prediction unit may be determined, and detailedinformation (e.g., an intra prediction mode, a motion vector, areference picture, etc.) according to each prediction method may bedetermined. Here, the processing unit subjected to prediction may bedifferent from the processing unit for which the prediction method anddetailed content is determined. For example, the prediction method, theprediction mode, etc. may be determined by the prediction unit, andprediction may be performed by the transform unit. A residual value(residual block) between the generated prediction block and an originalblock may be input to the transform module 130. Also, prediction modeinformation, motion vector information, etc. used for prediction may beencoded with the residual value by the entropy encoding module 165 andmay be transmitted to a device for decoding a video. When a particularencoding mode is used, it is possible to transmit to a device fordecoding video by encoding the original block as it is withoutgenerating the prediction block through the prediction modules 120 and125.

The inter prediction module 120 may predict the prediction unit based oninformation of at least one of a previous picture or a subsequentpicture of the current picture, or may predict the prediction unit basedon information of some encoded regions in the current picture, in somecases. The inter prediction module 120 may include a reference pictureinterpolation module, a motion prediction module, and a motioncompensation module.

The reference picture interpolation module may receive reference pictureinformation from the memory 155 and may generate pixel information of aninteger pixel or less then the integer pixel from the reference picture.In the case of luma pixels, an 8-tap DCT-based interpolation filterhaving different filter coefficients may be used to generate pixelinformation of an integer pixel or less than an integer pixel in unitsof a ¼ pixel. In the case of chroma signals, a 4-tap DCT-basedinterpolation filter having different filter coefficient may be used togenerate pixel information of an integer pixel or less than an integerpixel in units of a ⅛ pixel.

The motion prediction module may perform motion prediction based on thereference picture interpolated by the reference picture interpolationmodule. As methods for calculating a motion vector, various methods,such as a full search-based block matching algorithm (FBMA), a threestep search (TSS), a new three-step search algorithm (NTS), etc., may beused. The motion vector may have a motion vector value in units of a ½pixel or a ¼ pixel based on an interpolated pixel. The motion predictionmodule may predict a current prediction unit by changing the motionprediction method. As motion prediction methods, various methods, suchas a skip method, a merge method, an AMVP (Advanced Motion VectorPrediction) method, an intra block copy method, etc., may be used.

The intra prediction module 125 may generate a prediction unit based onreference pixel information neighboring to a current block which ispixel information in the current picture. When the neighboring block ofthe current prediction unit is a block subjected to inter prediction andthus a reference pixel is a pixel subjected to inter prediction, thereference pixel included in the block subjected to inter prediction maybe replaced with reference pixel information of a neighboring blocksubjected to intra prediction. That is, when a reference pixel is notavailable, at least one reference pixel of available reference pixelsmay be used instead of unavailable reference pixel information.

Prediction modes in intra prediction may include a directionalprediction mode using reference pixel information depending on aprediction direction and a non-directional prediction mode not usingdirectional information in performing prediction. A mode for predictingluma information may be different from a mode for predicting chromainformation, and in order to predict the chroma information, intraprediction mode information used to predict luma information orpredicted luma signal information may be utilized.

In performing intra prediction, when the size of the prediction unit isthe same as the size of the transform unit, intra prediction may beperformed on the prediction unit based on pixels positioned at the left,the top left, and the top of the prediction unit. However, in performingintra prediction, when the size of the prediction unit is different fromthe size of the transform unit, intra prediction may be performed usinga reference pixel based on the transform unit. Also, intra predictionusing N×N partitioning may be used for only the smallest coding unit.

In the intra prediction method, a prediction block may be generatedafter applying an AIS (Adaptive Intra Smoothing) filter to a referencepixel depending on the prediction modes. The type of the AIS filterapplied to the reference pixel may vary. In order to perform the intraprediction method, an intra prediction mode of the current predictionunit may be predicted from the intra prediction mode of the predictionunit neighboring to the current prediction unit. In prediction of theprediction mode of the current prediction unit by using mode informationpredicted from the neighboring prediction unit, when the intraprediction mode of the current prediction unit is the same as the intraprediction mode of the neighboring prediction unit, informationindicating that the prediction modes of the current prediction unit andthe neighboring prediction unit are equal to each other may betransmitted using predetermined flag information. When the predictionmode of the current prediction unit is different from the predictionmode of the neighboring prediction unit, entropy encoding may beperformed to encode prediction mode information of the current block.

Also, a residual block including information on a residual value whichis a different between the prediction unit subjected to prediction andthe original block of the prediction unit may be generated based onprediction units generated by the prediction modules 120 and 125. Thegenerated residual block may be input to the transform module 130.

The transform module 130 may transform the residual block including theinformation on the residual value between the original block and theprediction unit generated by the prediction modules 120 and 125 by usinga transform method, such as discrete cosine transform (DCT), discretesine transform (DST), and KLT. Whether to apply DCT, DST, or KLT inorder to transform the residual block may be determined based on intraprediction mode information of the prediction unit used to generate theresidual block.

The quantization module 135 may quantize values transformed to afrequency domain by the transform module 130. Quantization coefficientsmay vary depending on the block or importance of a picture. The valuescalculated by the quantization module 135 may be provided to the inversequantization module 140 and the rearrangement module 160.

The rearrangement module 160 may rearrange coefficients of quantizedresidual values.

The rearrangement module 160 may change a coefficient in the form of atwo-dimensional block into a coefficient in the form of aone-dimensional vector through a coefficient scanning method. Forexample, the rearrangement module 160 may scan from a DC coefficient toa coefficient in a high frequency domain using a zigzag scanning methodso as to change the coefficients to be in the form of one-dimensionalvectors. Depending on the size of the transform unit and the intraprediction mode, vertical direction scanning where coefficients in theform of two-dimensional blocks are scanned in the column direction orhorizontal direction scanning where coefficients in the form oftwo-dimensional blocks are scanned in the row direction may be usedinstead of zigzag scanning. That is, which scanning method among zigzagscanning, vertical direction scanning, and horizontal direction scanningis used may be determined depending on the size of the transform unitand the intra prediction mode.

The entropy encoding module 165 may perform entropy encoding based onthe values calculated by the rearrangement module 160. Entropy encodingmay use various encoding methods, for example, exponential Golombcoding, context-adaptive variable length coding (CAVLC), andcontext-adaptive binary arithmetic coding (CABAC).

The entropy encoding module 165 may encode a variety of information,such as residual value coefficient information and block typeinformation of the coding unit, prediction mode information, partitionunit information, prediction unit information, transform unitinformation, motion vector information, reference frame information,block interpolation information, filtering information, etc. from therearrangement module 160 and the prediction modules 120 and 125.

The entropy encoding module 165 may entropy encode the coefficients ofthe coding unit input from the rearrangement module 160.

The inverse quantization module 140 may inversely quantize the valuesquantized by the quantization module 135 and the inverse transformmodule 145 may inversely transform the values transformed by thetransform module 130. The residual value generated by the inversequantization module 140 and the inverse transform module 145 may becombined with the prediction unit predicted by a motion estimationmodule, a motion compensation module, and the intra prediction module ofthe prediction modules 120 and 125 such that a reconstructed block canbe generated.

The filter module 150 may include at least one of a deblocking filter,an offset correction unit, and an adaptive loop filter (ALF).

The deblocking filter may remove block distortion that occurs due toboundaries between the blocks in the reconstructed picture. In order todetermine whether to perform deblocking, the pixels included in severalrows or columns in the block may be a basis of determining whether toapply the deblocking filter to the current block. When the deblockingfilter is applied to the block, a strong filter or a weak filter may beapplied depending on required deblocking filtering strength. Also, inapplying the deblocking filter, horizontal direction filtering andvertical direction filtering may be processed in parallel.

The offset correction module may correct offset with the originalpicture in units of a pixel in the picture subjected to deblocking. Inorder to perform the offset correction on a particular picture, it ispossible to use a method of applying offset in consideration of edgeinformation of each pixel or a method of partitioning pixels of apicture into the predetermined number of regions, determining a regionto be subjected to perform offset, and applying the offset to thedetermined region.

Adaptive loop filtering (ALF) may be performed based on the valueobtained by comparing the filtered reconstructed picture and theoriginal picture. The pixels included in the picture may be divided intopredetermined groups, a filter to be applied to each of the groups maybe determined, and filtering may be individually performed for eachgroup. Information on whether to apply ALF and a luma signal may betransmitted by coding units (CU). The shape and filter coefficient of afilter for ALF may vary depending on each block. Also, the filter forALF in the same shape (fixed shape) may be applied regardless ofcharacteristics of the application target block.

The memory 155 may store the reconstructed block or picture calculatedthrough the filter module 150. The stored reconstructed block or picturemay be provided to the prediction modules 120 and 125 in performinginter prediction.

FIG. 2 is a block diagram illustrating a device for decoding a videoaccording to an embodiment of the present invention.

Referring to FIG. 2 , the device 200 for decoding a video may include:an entropy decoding module 210, a rearrangement module 215, an inversequantization module 220, an inverse transform module 225, predictionmodules 230 and 235, a filter module 240, and a memory 245.

When a video bitstream is input from the device for encoding a video,the input bitstream may be decoded according to an inverse process ofthe device for encoding a video.

The entropy decoding module 210 may perform entropy decoding accordingto an inverse process of entropy encoding by the entropy encoding moduleof the device for encoding a video. For example, corresponding to themethods performed by the device for encoding a video, various methods,such as exponential Golomb coding, context-adaptive variable lengthcoding (CAVLC), and context-adaptive binary arithmetic coding (CABAC)may be applied.

The entropy decoding module 210 may decode information on intraprediction and inter prediction performed by the device for encoding avideo.

The rearrangement module 215 may perform rearrangement on the bitstreamentropy decoded by the entropy decoding module 210 based on therearrangement method used in the device for encoding a video. Therearrangement module may reconstruct and rearrange the coefficients inthe form of one-dimensional vectors to the coefficient in the form oftwo-dimensional blocks. The rearrangement module 215 may receiveinformation related to coefficient scanning performed in the device forencoding a video and may perform rearrangement via a method of inverselyscanning the coefficients based on the scanning order performed in thedevice for encoding a video.

The inverse quantization module 220 may perform inverse quantizationbased on a quantization parameter received from the device for encodinga video and the rearranged coefficients of the block.

The inverse transform module 225 may perform the inverse transform,i.e., inverse DCT, inverse DST, and inverse KLT, which is the inverseprocess of transform, i.e., DCT, DST, and KLT, performed by thetransform module on the quantization result by the device for encoding avideo. Inverse transform may be performed based on a transfer unitdetermined by the device for encoding a video. The inverse transformmodule 225 of the device for decoding a video may selectively performtransform schemes (e.g., DCT, DST, and KLT) depending on multiple piecesof information, such as the prediction method, the size of the currentblock, the prediction direction, etc.

The prediction modules 230 and 235 may generate a prediction block basedon information on prediction block generation received from the entropydecoding module 210 and previously decoded block or picture informationreceived from the memory 245.

As described above, like the operation of the device for encoding avideo, in performing intra prediction, when the size of the predictionunit is the same as the size of the transform unit, intra prediction maybe performed on the prediction unit based on the pixels positioned atthe left, the top left, and the top of the prediction unit. Inperforming intra prediction, when the size of the prediction unit isdifferent from the size of the transform unit, intra prediction may beperformed using a reference pixel based on the transform unit. Also,intra prediction using N×N partitioning may be used for only thesmallest coding unit.

The prediction modules 230 and 235 may include a prediction unitdetermination module, an inter prediction module, and an intraprediction module. The prediction unit determination module may receivea variety of information, such as prediction unit information,prediction mode information of an intra prediction method, informationon motion prediction of an inter prediction method, etc. from theentropy decoding module 210, may divide a current coding unit intoprediction units, and may determine whether inter prediction or intraprediction is performed on the prediction unit. By using informationrequired in inter prediction of the current prediction unit receivedfrom the device for encoding a video, the inter prediction module 230may perform inter prediction on the current prediction unit based oninformation of at least one of a previous picture or a subsequentpicture of the current picture including the current prediction unit.Alternatively, inter prediction may be performed based on information ofsome pre-reconstructed regions in the current picture including thecurrent prediction unit.

In order to perform inter prediction, it may be determined for thecoding unit which of a skip mode, a merge mode, an AMVP mode, and aninter block copy mode is used as the motion prediction method of theprediction unit included in the coding unit.

The intra prediction module 235 may generate a prediction block based onpixel information in the current picture. When the prediction unit is aprediction unit subjected to intra prediction, intra prediction may beperformed based on intra prediction mode information of the predictionunit received from the device for encoding a video. The intra predictionmodule 235 may include an adaptive intra smoothing (AIS) filter, areference pixel interpolation module, and a DC filter. The AIS filterperforms filtering on the reference pixel of the current block, andwhether to apply the filter may be determined depending on theprediction mode of the current prediction unit. AIS filtering may beperformed on the reference pixel of the current block by using theprediction mode of the prediction unit and AIS filter informationreceived from the device for encoding a video. When the prediction modeof the current block is a mode where AIS filtering is not performed, theAIS filter may not be applied.

When the prediction mode of the prediction unit is a prediction mode inwhich intra prediction is performed based on the pixel value obtained byinterpolating the reference pixel, the reference pixel interpolationmodule may interpolate the reference pixel to generate the referencepixel of an integer pixel or less than an integer pixel. When theprediction mode of the current prediction unit is a prediction mode inwhich a prediction block is generated without interpolation thereference pixel, the reference pixel may not be interpolated. The DCfilter may generate a prediction block through filtering when theprediction mode of the current block is a DC mode.

The reconstructed block or picture may be provided to the filter module240. The filter module 240 may include the deblocking filter, the offsetcorrection module, and the ALF.

Information on whether or not the deblocking filter is applied to thecorresponding block or picture and information on which of a strongfilter and a weak filter is applied when the deblocking filter isapplied may be received from the device for encoding a video. Thedeblocking filter of the device for decoding a video may receiveinformation on the deblocking filter from the device for encoding avideo, and may perform deblocking filtering on the corresponding block.

The offset correction module may perform offset correction on thereconstructed picture based on the type of offset correction and offsetvalue information applied to a picture in performing encoding.

The ALF may be applied to the coding unit based on information onwhether to apply the ALF, ALF coefficient information, etc. receivedfrom the device for encoding a video. The ALF information may beprovided as being included in a particular parameter set.

The memory 245 may store the reconstructed picture or block for use as areference picture or block, and may provide the reconstructed picture toan output module.

As described above, in the embodiment of the present invention, forconvenience of explanation, the coding unit is used as a termrepresenting a unit for encoding, but the coding unit may serve as aunit performing decoding as well as encoding.

In addition, a current block may represent a target block to beencoded/decoded. And, the current block may represent a coding treeblock (or a coding tree unit), a coding block (or a coding unit), atransform block (or a transform unit), a prediction block (or aprediction unit), or the like depending on an encoding/decoding step.

A picture may be encoded/decoded by divided into base blocks having asquare shape or a non-square shape. At this time, the base block may bereferred to as a coding tree unit. The coding tree unit may be definedas a coding unit of the largest size allowed within a sequence or aslice. Information regarding whether the coding tree unit has a squareshape or has a non-square shape or information regarding a size of thecoding tree unit may be signaled through a sequence parameter set, apicture parameter set, or a slice header. The coding tree unit may bedivided into smaller size partitions. At this time, if it is assumedthat a depth of a partition generated by dividing the coding tree unitis 1, a depth of a partition generated by dividing the partition havingdepth 1 may be defined as 2. That is, a partition generated by dividinga partition having a depth k in the coding tree unit may be defined ashaving a depth k+1.

A partition of arbitrary size generated by dividing a coding tree unitmay be defined as a coding unit. The coding unit may be recursivelydivided or divided into base units for performing prediction,quantization, transform, or in-loop filtering, and the like. Forexample, a partition of arbitrary size generated by dividing the codingunit may be defined as a coding unit, or may be defined as a transformunit or a prediction unit, which is a base unit for performingprediction, quantization, transform or in-loop filtering and the like.

Partitioning of a coding tree unit or a coding unit may be performedbased on at least one of a vertical line and a horizontal line. Inaddition, the number of vertical lines or horizontal lines partitioningthe coding tree unit or the coding unit may be at least one or more. Forexample, the coding tree unit or the coding unit may be divided into twopartitions using one vertical line or one horizontal line, or the codingtree unit or the coding unit may be divided into three partitions usingtwo vertical lines or two horizontal lines. Alternatively, the codingtree unit or the coding unit may be partitioned into four partitionshaving a length and a width of ½ by using one vertical line and onehorizontal line.

When a coding tree unit or a coding unit is divided into a plurality ofpartitions using at least one vertical line or at least one horizontalline, the partitions may have a uniform size or a different size.Alternatively, any one partition may have a different size from theremaining partitions.

In the embodiments described below, it is assumed that a coding treeunit or a coding unit is divided into a quad tree structure or a binarytree structure. However, it is also possible to divide a coding treeunit or a coding unit using a larger number of vertical lines or alarger number of horizontal lines.

FIG. 3 is a diagram illustrating an example of hierarchicallypartitioning a coding block based on a tree structure according to anembodiment of the present invention.

An input video signal is decoded in predetermined block units. Such adefault unit for decoding the input video signal is a coding block. Thecoding block may be a unit performing intra/inter prediction, transform,and quantization. In addition, a prediction mode (e.g., intra predictionmode or inter prediction mode) is determined in units of a coding block,and the prediction blocks included in the coding block may share thedetermined prediction mode. The coding block may be a square ornon-square block having an arbitrary size in a range of 8×8 to 64×64, ormay be a square or non-square block having a size of 128×128, 256×256,or more.

Specifically, the coding block may be hierarchically partitioned basedon at least one of a quad tree and a binary tree. Here, quad tree-basedpartitioning may mean that a 2N×2N coding block is partitioned into fourN×N coding blocks, and binary tree-based partitioning may mean that onecoding block is partitioned into two coding blocks. Even if the binarytree-based partitioning is performed, a square-shaped coding block mayexist in the lower depth.

Binary tree-based partitioning may be symmetrically or asymmetricallyperformed. The coding block partitioned based on the binary tree may bea square block or a non-square block, such as a rectangular shape. Forexample, a partition type in which the binary tree-based partitioning isallowed may comprise at least one of a symmetric type of 2N×N(horizontal directional non-square coding unit) or N×2N (verticaldirection non-square coding unit), asymmetric type of nL×2N, nR×2N,2N×nU, or 2N×nD.

Binary tree-based partitioning may be limitedly allowed to one of asymmetric or an asymmetric type partition. In this case, constructingthe coding tree unit with square blocks may correspond to quad tree CUpartitioning, and constructing the coding tree unit with symmetricnon-square blocks may correspond to binary tree partitioning.Constructing the coding tree unit with square blocks and symmetricnon-square blocks may correspond to quad and binary tree CUpartitioning.

Binary tree-based partitioning may be performed on a coding block wherequad tree-based partitioning is no longer performed. Quad tree-basedpartitioning may no longer be performed on the coding block partitionedbased on the binary tree.

Furthermore, partitioning of a lower depth may be determined dependingon a partition type of an upper depth. For example, if binary tree-basedpartitioning is allowed in two or more depths, only the same type as thebinary tree partitioning of the upper depth may be allowed in the lowerdepth. For example, if the binary tree-based partitioning in the upperdepth is performed with 2N×N type, the binary tree-based partitioning inthe lower depth is also performed with 2N×N type. Alternatively, if thebinary tree-based partitioning in the upper depth is performed with N×2Ntype, the binary tree-based partitioning in the lower depth is alsoperformed with N×2N type.

On the contrary, it is also possible to allow, in a lower depth, only atype different from a binary tree partitioning type of an upper depth.

It may be possible to limit only a specific type of binary tree basedpartitioning to be used for sequence, slice, coding tree unit, or codingunit. As an example, only 2N×N type or N×2N type of binary tree-basedpartitioning may be allowed for the coding tree unit. An availablepartition type may be predefined in an encoder or a decoder. Orinformation on available partition type or on unavailable partition typeon may be encoded and then signaled through a bitstream.

FIGS. 5A and 5B are diagrams illustrating an example in which only aspecific type of binary tree-based partitioning is allowed. FIG. 5Ashows an example in which only N×2N type of binary tree-basedpartitioning is allowed, and FIG. 5B shows an example in which only 2N×Ntype of binary tree-based partitioning is allowed. In order to implementadaptive partitioning based on the quad tree or binary tree, informationindicating quad tree-based partitioning, information on the size/depthof the coding block that quad tree-based partitioning is allowed,information indicating binary tree-based partitioning, information onthe size/depth of the coding block that binary tree-based partitioningis allowed, information on the size/depth of the coding block thatbinary tree-based partitioning is not allowed, information on whetherbinary tree-based partitioning is performed in a vertical direction or ahorizontal direction, etc. may be used.

In addition, information on the number of times a binary treepartitioning is allowed, a depth at which the binary tree partitioningis allowed, or the number of the depths at which the binary treepartitioning is allowed may be obtained for a coding tree unit or aspecific coding unit. The information may be encoded in units of acoding tree unit or a coding unit, and may be transmitted to a decoderthrough a bitstream.

For example, a syntax ‘max_binary_depth_idx_minus1’ indicating a maximumdepth at which binary tree partitioning is allowed may beencoded/decoded through a bitstream. In this case,max_binary_depth_idx_minus1+1 may indicate the maximum depth at whichthe binary tree partitioning is allowed.

Referring to the example shown in FIG. 6 , in FIG. 6 , the binary treepartitioning has been performed for a coding unit having a depth of 2and a coding unit having a depth of 3. Accordingly, at least one ofinformation indicating the number of times the binary tree partitioningin the coding tree unit has been performed (i.e., 2 times), informationindicating the maximum depth which the binary tree partitioning has beenallowed in the coding tree unit (i.e., depth 3), or the number of depthsin which the binary tree partitioning has been performed in the codingtree unit (i.e., 2 (depth 2 and depth 3)) may be encoded/decoded througha bitstream.

As another example, at least one of information on the number of timesthe binary tree partitioning is permitted, the depth at which the binarytree partitioning is allowed, or the number of the depths at which thebinary tree partitioning is allowed may be obtained for each sequence oreach slice. For example, the information may be encoded in units of asequence, a picture, or a slice unit and transmitted through abitstream. Accordingly, at least one of the number of the binary treepartitioning in a first slice, the maximum depth in which the binarytree partitioning is allowed in the first slice, or the number of depthsin which the binary tree partitioning is performed in the first slicemay be difference from a second slice. For example, in the first slice,binary tree partitioning may be permitted for only one depth, while inthe second slice, binary tree partitioning may be permitted for twodepths.

As another example, the number of times the binary tree partitioning ispermitted, the depth at which the binary tree partitioning is allowed,or the number of depths at which the binary tree partitioning is allowedmay be set differently according to a time level identifier (TemporalID)of a slice or a picture. Here, the temporal level identifier(TemporalID) is used to identify each of a plurality of layers of videohaving a scalability of at least one of view, spatial, temporal orquality.

As shown in FIG. 3 , the first coding block 300 with the partition depth(split depth) of k may be partitioned into multiple second coding blocksbased on the quad tree. For example, the second coding blocks 310 to 340may be square blocks having the half width and the half height of thefirst coding block, and the partition depth of the second coding blockmay be increased to k+1.

The second coding block 310 with the partition depth of k+1 may bepartitioned into multiple third coding blocks with the partition depthof k+2. Partitioning of the second coding block 310 may be performed byselectively using one of the quad tree and the binary tree depending ona partitioning method. Here, the partitioning method may be determinedbased on at least one of the information indicating quad tree-basedpartitioning and the information indicating binary tree-basedpartitioning.

When the second coding block 310 is partitioned based on the quad tree,the second coding block 310 may be partitioned into four third codingblocks 310 a having the half width and the half height of the secondcoding block, and the partition depth of the third coding block 310 amay be increased to k+2. In contrast, when the second coding block 310is partitioned based on the binary tree, the second coding block 310 maybe partitioned into two third coding blocks. Here, each of two thirdcoding blocks may be a non-square block having one of the half width andthe half height of the second coding block, and the partition depth maybe increased to k+2. The second coding block may be determined as anon-square block of a horizontal direction or a vertical directiondepending on a partitioning direction, and the partitioning directionmay be determined based on the information on whether binary tree-basedpartitioning is performed in a vertical direction or a horizontaldirection.

In the meantime, the second coding block 310 may be determined as a leafcoding block that is no longer partitioned based on the quad tree or thebinary tree. In this case, the leaf coding block may be used as aprediction block or a transform block.

Like partitioning of the second coding block 310, the third coding block310 a may be determined as a leaf coding block, or may be furtherpartitioned based on the quad tree or the binary tree.

In the meantime, the third coding block 310 b partitioned based on thebinary tree may be further partitioned into coding blocks 310 b-2 of avertical direction or coding blocks 310 b-3 of a horizontal directionbased on the binary tree, and the partition depth of the relevant codingblocks may be increased to k+3. Alternatively, the third coding block310 b may be determined as a leaf coding block 310 b-1 that is no longerpartitioned based on the binary tree. In this case, the coding block 310b-1 may be used as a prediction block or a transform block. However, theabove partitioning process may be limitedly performed based on at leastone of the information on the size/depth of the coding block that quadtree-based partitioning is allowed, the information on the size/depth ofthe coding block that binary tree-based partitioning is allowed, and theinformation on the size/depth of the coding block that binary tree-basedpartitioning is not allowed.

A number of a candidate that represent a size of a coding block may belimited to a predetermined number, or a size of a coding block in apredetermined unit may have a fixed value. As an example, the size ofthe coding block in a sequence or in a picture may be limited to have256×256, 128×128, or 32×32. Information indicating the size of thecoding block in the sequence or in the picture may be signaled through asequence header or a picture header.

As a result of partitioning based on a quad tree and a binary tree, acoding unit may be represented as square or rectangular shape of anarbitrary size.

A coding block is encoded using at least one of a skip mode, intraprediction, inter prediction, or a skip method. Once a coding block isdetermined, a prediction block may be determined through predictivepartitioning of the coding block. The predictive partitioning of thecoding block may be performed by a partition mode (Part mode) indicatinga partition type of the coding block. A size or a shape of theprediction block may be determined according to the partition mode ofthe coding block. For example, a size of a prediction block determinedaccording to the partition mode may be equal to or smaller than a sizeof a coding block.

FIG. 7 is a diagram illustrating a partition mode that may be applied toa coding block when the coding block is encoded by inter prediction.

When a coding block is encoded by inter prediction, one of 8partitioning modes may be applied to the coding block, as in the exampleshown in FIG. 7 .

When a coding block is encoded by intra prediction, a partition modePART_2N×2N or a partition mode PART_N×N may be applied to the codingblock.

PART_N×N may be applied when a coding block has a minimum size. Here,the minimum size of the coding block may be pre-defined in an encoderand a decoder. Or, information regarding the minimum size of the codingblock may be signaled via a bitstream. For example, the minimum size ofthe coding block may be signaled through a slice header, so that theminimum size of the coding block may be defined per slice.

In general, a prediction block may have a size from 64×64 to 4×4.However, when a coding block is encoded by inter prediction, it may berestricted that the prediction block does not have a 4×4 size in orderto reduce memory bandwidth when performing motion compensation.

FIG. 8 is a diagram illustrating types of pre-defined intra predictionmodes for a device for encoding/decoding a video according to anembodiment of the present invention.

The device for encoding/decoding a video may perform intra predictionusing one of pre-defined intra prediction modes. The pre-defined intraprediction modes for intra prediction may include non-directionalprediction modes (e.g., a planar mode, a DC mode) and 33 directionalprediction modes.

Alternatively, in order to enhance accuracy of intra prediction, alarger number of directional prediction modes than the 33 directionalprediction modes may be used. That is, M extended directional predictionmodes may be defined by subdividing angles of the directional predictionmodes (M>33), and a directional prediction mode having a predeterminedangle may be derived using at least one of the 33 pre-defineddirectional prediction modes.

A larger number of intra prediction modes than 35 intra prediction modesshown in FIG. 8 may be used. For example, a larger number of intraprediction modes than the 35 intra prediction modes can be used bysubdividing angles of directional prediction modes or by deriving adirectional prediction mode having a predetermined angle using at leastone of a pre-defined number of directional prediction modes. At thistime, the use of a larger number of intra prediction modes than the 35intra prediction modes may be referred to as an extended intraprediction mode.

FIG. 9 shows an example of extended intra prediction modes, and theextended intra prediction modes may include two non-directionalprediction modes and 65 extended directional prediction modes. The samenumbers of the extended intra prediction modes may be used for a lumacomponent and a chroma component, or a different number of intraprediction modes may be used for each component. For example, 67extended intra prediction modes may be used for the luma component, and35 intra prediction modes may be used for the chroma component.

Alternatively, depending on the chroma format, a different number ofintra prediction modes may be used in performing intra prediction. Forexample, in the case of the 4:2:0 format, 67 intra prediction modes maybe used for the luma component to perform intra prediction and 35 intraprediction modes may be used for the chroma component. In the case ofthe 4:4:4 format, 67 intra prediction modes may be used for both theluma component and the chroma component to perform intra prediction.

Alternatively, depending on the size and/or shape of the block, adifferent number of intra prediction modes may be used to perform intraprediction. That is, depending on the size and/or shape of the PU or CU,35 intra prediction modes or 67 intra prediction modes may be used toperform intra prediction. For example, when the CU or PU has the sizeless than 64×64 or is asymmetrically partitioned, 35 intra predictionmodes may be used to perform intra prediction. When the size of the CUor PU is equal to or greater than 64×64, 67 intra prediction modes maybe used to perform intra prediction. 65 directional intra predictionmodes may be allowed for Intra_2N×2N, and only 35 directional intraprediction modes may be allowed for Intra_N×N.

A size of a block to which the extended intra prediction mode is appliedmay be set differently for each sequence, picture or slice. For example,it is set that the extended intra prediction mode is applied to a block(e.g., CU or PU) which has a size greater than 64×64 in the first slice.On the other hands, it is set that the extended intra prediction mode isapplied to a block which has a size greater than 32×32 in the secondslice. Information representing a size of a block to which the extendedintra prediction mode is applied may be signaled through in units of asequence, a picture, or a slice. For example, the information indicatingthe size of the block to which the extended intra prediction mode isapplied may be defined as ‘log_2_extended_intra_mode_size_minus4’obtained by taking a logarithm of the block size and then subtractingthe integer 4. For example, if a value oflog_2_extended_intra_mode_size_minus4 is 0, it may indicate that theextended intra prediction mode may be applied to a block having a sizeequal to or greater than 16×16. And if a value oflog_2_extended_intra_mode_size_minus4 is 1, it may indicate that theextended intra prediction mode may be applied to a block having a sizeequal to or greater than 32×32.

As described above, the number of intra prediction modes may bedetermined in consideration of at least one of a color component, achroma format, and a size or a shape of a block. In addition, the numberof intra prediction mode candidates (e.g., the number of MPMs) used fordetermining an intra prediction mode of a current block to beencoded/decoded may also be determined according to at least one of acolor component, a color format, and the size or a shape of a block. Amethod of determining an intra prediction mode of a current block to beencoded/decoded and a method of performing intra prediction using thedetermined intra prediction mode will be described with the drawings.

FIG. 10 is a flowchart briefly illustrating an intra prediction methodaccording to an embodiment of the present invention.

Encoding/decoding efficiency may be improved by applying an intraprediction to a block included in an image having strong directionality,a block included in an image not shown in a previous frame, or the like.

Referring to FIG. 10 , an intra prediction mode of the current block maybe determined at step S1000.

Specifically, the intra prediction mode of the current block may bederived based on a candidate list and an index. Here, the candidate listcontains multiple candidates, and the multiple candidates may bedetermined based on an intra prediction mode of the neighboring blockadjacent to the current block. The neighboring block may include atleast one of blocks positioned at the top, the bottom, the left, theright, and the corner of the current block. The index may specify one ofthe multiple candidates of the candidate list. The candidate specifiedby the index may be set to the intra prediction mode of the currentblock.

An intra prediction mode used for intra prediction in the neighboringblock may be set as a candidate. Also, an intra prediction mode havingdirectionality similar to that of the intra prediction mode of theneighboring block may be set as a candidate. Here, the intra predictionmode having similar directionality may be determined by adding orsubtracting a predetermined constant value to or from the intraprediction mode of the neighboring block. The predetermined constantvalue may be an integer, such as one, two, or more.

The candidate list may further include a default mode. The default modemay include at least one of a planar mode, a DC mode, a vertical mode,and a horizontal mode. The default mode may be adaptively addedconsidering the maximum number of candidates that can be included in thecandidate list of the current block.

The maximum number of candidates that can be included in the candidatelist may be three, four, five, six, or more. The maximum number ofcandidates that can be included in the candidate list may be a fixedvalue preset in the device for encoding/decoding a video, or may bevariably determined based on a characteristic of the current block. Thecharacteristic may mean the location/size/shape of the block, thenumber/type of intra prediction modes that the block can use, a colortype, a color format, etc. Alternatively, information indicating themaximum number of candidates that can be included in the candidate listmay be signaled separately, and the maximum number of candidates thatcan be included in the candidate list may be variably determined usingthe information. The information indicating the maximum number ofcandidates may be signaled in at least one of a sequence level, apicture level, a slice level, and a block level.

When the extended intra prediction modes and the 35 pre-defined intraprediction modes are selectively used, the intra prediction modes of theneighboring blocks may be transformed into indexes corresponding to theextended intra prediction modes, or into indexes corresponding to the 35intra prediction modes, whereby candidates can be derived. For transformto an index, a pre-defined table may be used, or a scaling operationbased on a predetermined value may be used. Here, the pre-defined tablemay define a mapping relation between different intra prediction modegroups (e.g., extended intra prediction modes and 35 intra predictionmodes).

For example, when the left neighboring block uses the 35 intraprediction modes and the intra prediction mode of the left neighboringblock is 10 (a horizontal mode), it may be transformed into an index of16 corresponding to a horizontal mode in the extended intra predictionmodes.

Alternatively, when the top neighboring block uses the extended intraprediction modes and the intra prediction mode the top neighboring blockhas an index of 50 (a vertical mode), it may be transformed into anindex of 26 corresponding to a vertical mode in the 35 intra predictionmodes.

Based on the above-described method of determining the intra predictionmode, the intra prediction mode may be derived independently for each ofthe luma component and the chroma component, or the intra predictionmode of the chroma component may be derived depending on the intraprediction mode of the luma component.

Specifically, the intra prediction mode of the chroma component may bedetermined based on the intra prediction mode of the luma component asshown in the following Table 1.

TABLE 1 Intra_chroma_pred_mode IntraPredModeY[xCb][yCb] [xCb][yCb] 0 2610 1 X(0 <= X <= 34) 0 34 0 0 0 0 1 26 34 26 26 26 2 10 10 34 10 10 3 11 1 34 1 4 0 26 10 1 X

In Table 1, intra_chroma_pred_mode means information signaled to specifythe intra prediction mode of the chroma component, and IntraPredModeYindicates the intra prediction mode of the luma component.

Referring to FIG. 10 , a reference sample for intra prediction of thecurrent block may be derived at step S1010.

Specifically, a reference sample for intra prediction may be derivedbased on a neighboring sample of the current block. The neighboringsample may be a reconstructed sample of the neighboring block, and thereconstructed sample may be a reconstructed sample before an in-loopfilter is applied or a reconstructed sample after the in-loop filter isapplied.

A neighboring sample reconstructed before the current block may be usedas the reference sample, and a neighboring sample filtered based on apredetermined intra filter may be used as the reference sample.Filtering of neighboring samples using an intra filter may also bereferred to as reference sample smoothing. The intra filter may includeat least one of the first intra filter applied to multiple neighboringsamples positioned on the same horizontal line and the second intrafilter applied to multiple neighboring samples positioned on the samevertical line. Depending on the positions of the neighboring samples,one of the first intra filter and the second intra filter may beselectively applied, or both intra filters may be applied. At this time,at least one filter coefficient of the first intra filter or the secondintra filter may be (1, 2, 1), but is not limited thereto.

Filtering may be adaptively performed based on at least one of the intraprediction mode of the current block and the size of the transform blockfor the current block. For example, when the intra prediction mode ofthe current block is the DC mode, the vertical mode, or the horizontalmode, filtering may not be performed. When the size of the transformblock is N×M, filtering may not be performed. Here, N and M may be thesame values or different values, or may be values of 4, 8, 16, or more.For example, if the size of the transform block is 4×4, filtering maynot be performed. Alternatively, filtering may be selectively performedbased on the result of a comparison of a pre-defined threshold and thedifference between the intra prediction mode of the current block andthe vertical mode (or the horizontal mode). For example, when thedifference between the intra prediction mode of the current block andthe vertical mode is greater than a threshold, filtering may beperformed. The threshold may be defined for each size of the transformblock as shown in Table 2.

TABLE 2 8 × 8 transform 16 × 16 transform 32 × 32 transform Threshold 71 0

The intra filter may be determined as one of multiple intra filtercandidates pre-defined in the device for encoding/decoding a video. Tothis end, an index specifying an intra filter of the current block amongthe multiple intra filter candidates may be signaled. Alternatively, theintra filter may be determined based on at least one of the size/shapeof the current block, the size/shape of the transform block, informationon the filter strength, and variations of the neighboring samples.

Referring to FIG. 10 , intra prediction may be performed using the intraprediction mode of the current block and the reference sample at stepS1020.

That is, the prediction sample of the current block may be obtainedusing the intra prediction mode determined at step S1000 and thereference sample derived at step S1010. However, in the case of intraprediction, a boundary sample of the neighboring block may be used, andthus quality of the prediction picture may be decreased. Therefore, acorrection process may be performed on the prediction sample generatedthrough the above-described prediction process, and will be described indetail with reference to FIGS. 11 to 13 . However, the correctionprocess is not limited to being applied only to the intra predictionsample, and may be applied to an inter prediction sample or thereconstructed sample.

FIG. 11 is a diagram illustrating a method of correcting a predictionsample of a current block based on differential information ofneighboring samples according to an embodiment of the present invention.

The prediction sample of the current block may be corrected based on thedifferential information of multiple neighboring samples for the currentblock. The correction may be performed on all prediction samples in thecurrent block, or may be performed on prediction samples inpredetermined partial regions. The partial regions may be one row/columnor multiple rows/columns, and these may be preset regions for correctionin the device for encoding/decoding a video. For example, correction maybe performed on a one row/column located at a boundary of the currentblock or may be performed on plurality of rows/columns from a boundaryof the current block. Alternatively, the partial regions may be variablydetermined based on at least one of the size/shape of the current blockand the intra prediction mode.

The neighboring samples may belong to the neighboring blocks positionedat the top, the left, and the top left corner of the current block. Thenumber of neighboring samples used for correction may be two, three,four, or more. The positions of the neighboring samples may be variablydetermined depending on the position of the prediction sample which isthe correction target in the current block. Alternatively, some of theneighboring samples may have fixed positions regardless of the positionof the prediction sample which is the correction target, and theremaining neighboring samples may have variable positions depending onthe position of the prediction sample which is the correction target.

The differential information of the neighboring samples may mean adifferential sample between the neighboring samples, or may mean a valueobtained by scaling the differential sample by a predetermined constantvalue (e.g., one, two, three, etc.). Here, the predetermined constantvalue may be determined considering the position of the predictionsample which is the correction target, the position of the column or rowincluding the prediction sample which is the correction target, theposition of the prediction sample within the column or row, etc.

For example, when the intra prediction mode of the current block is thevertical mode, differential samples between the top left neighboringsample p(−1, −1) and neighboring samples p (−1, y) adjacent to the leftboundary of the current block may be used to obtain the final predictionsample as shown in Equation 1.P′(0,y)=P(0,y)+((p(−1,y)−p(−1,−1))>>1 for y=0 . . . N−1  [Equation 1]

For example, when the intra prediction mode of the current block is thehorizontal mode, differential samples between the top left neighboringsample p(−1, −1) and neighboring samples p(x, −1) adjacent to the topboundary of the current block may be used to obtain the final predictionsample as shown in Equation 2.P′(x,0)=p(x,0)+((p(x,−1)−p(−1,−1))>>1 for x=0 . . . N−1  [Equation 2]

For example, when the intra prediction mode of the current block is thevertical mode, differential samples between the top left neighboringsample p(−1, −1) and neighboring samples p(−1, y) adjacent to the leftboundary of the current block may be used to obtain the final predictionsample. Here, the differential sample may be added to the predictionsample, or the differential sample may be scaled by a predeterminedconstant value, and then added to the prediction sample. Thepredetermined constant value used in scaling may be determineddifferently depending on the column and/or row. For example, theprediction sample may be corrected as shown in Equation 3 and Equation4.P′(0,y)=P(0,y)+((p(−1,y)−p(−1,−1))>>1 for y=0 . . . N−1  [Equation 3]P′(1,y)=P(1,y)+((p(−1,y)−p(−1,−1))>>2 for y=0 . . . N−1  [Equation 4]

For example, when the intra prediction mode of the current block is thehorizontal mode, differential samples between the top left neighboringsample p(−1, −1) and neighboring samples p(x, −1) adjacent to the topboundary of the current block may be used to obtain the final predictionsample, as described in the case of the vertical mode. For example, theprediction sample may be corrected as shown in Equation 5 and Equation6.P′(x,0)=p(x,0)+((p(x,−1)−p(−1,−1))>>1 for x=0 . . . N−1  [Equation 5]P′(x,1)=p(x,1)+((p(x,−1)−p(−1,−1))>>2 for x=0 . . . N−1  [Equation 6]

FIGS. 12 and 13 are diagrams illustrating a method of correcting aprediction sample based on a predetermined correction filter accordingto an embodiment of the present invention.

The prediction sample may be corrected based on the neighboring sampleof the prediction sample which is the correction target and apredetermined correction filter. Here, the neighboring sample may bespecified by an angular line of the directional prediction mode of thecurrent block, or may be at least one sample positioned on the sameangular line as the prediction sample which is the correction target.Also, the neighboring sample may be a prediction sample in the currentblock, or may be a reconstructed sample in a neighboring blockreconstructed before the current block.

At least one of the number of taps, strength, and a filter coefficientof the correction filter may be determined based on at least one of theposition of the prediction sample which is the correction target,whether or not the prediction sample which is the correction target ispositioned on the boundary of the current block, the intra predictionmode of the current block, angle of the directional prediction mode, theprediction mode (inter or intra mode) of the neighboring block, and thesize/shape of the current block.

Referring to FIG. 12 , when the directional prediction mode has an indexof 2 or 34, at least one prediction/reconstructed sample positioned atthe bottom left of the prediction sample which is the correction targetand the predetermined correction filter may be used to obtain the finalprediction sample. Here, the prediction/reconstructed sample at thebottom left may belong to a previous line of a line including theprediction sample which is the correction target. Theprediction/reconstructed sample at the bottom left may belong to thesame block as the current sample, or to neighboring block adjacent tothe current block.

Filtering for the prediction sample may be performed only on the linepositioned at the block boundary, or may be performed on multiple lines.The correction filter where at least one of the number of filter tapsand a filter coefficient is different for each of lines may be used. Forexample, a (½, ½) filter may be used for the left first line closest tothe block boundary, a ( 12/16, 4/16) filter may be used for the secondline, a ( 14/16, 2/16) filter may be used for the third line, and a (15/16, 1/16) filter may be used for the fourth line.

Alternatively, when the directional prediction mode has an index of 3 to6 or 30 to 33, filtering may be performed on the block boundary as shownin FIG. 13 , and a 3-tap correction filter may be used to correct theprediction sample. Filtering may be performed using the bottom leftsample of the prediction sample which is the correction target, thebottom sample of the bottom left sample, and a 3-tap correction filterthat takes as input the prediction sample which is the correctiontarget. The position of neighboring sample used by the correction filtermay be determined differently based on the directional prediction mode.The filter coefficient of the correction filter may be determineddifferently depending on the directional prediction mode.

Different correction filters may be applied depending on whether theneighboring block is encoded in the inter mode or the intra mode. Whenthe neighboring block is encoded in the intra mode, a filtering methodwhere more weight is given to the prediction sample may be used,compared to when the neighboring block is encoded in the inter mode. Forexample, in the case of that the intra prediction mode is 34, when theneighboring block is encoded in the inter mode, a (½, ½) filter may beused, and when the neighboring block is encoded in the intra mode, a (4/16, 12/16) filter may be used.

The number of lines to be filtered in the current block may varydepending on the size/shape of the current block (e.g., the coding blockor the prediction block). For example, when the size of the currentblock is equal to or less than 32×32, filtering may be performed on onlyone line at the block boundary; otherwise, filtering may be performed onmultiple lines including the one line at the block boundary.

FIGS. 12 and 13 are based on the case where the 35 intra predictionmodes in FIG. 7 are used, but may be equally/similarly applied to thecase where the extended intra prediction modes are used.

FIG. 14 shows a range of reference samples for intra predictionaccording to an embodiment to which the present invention is applied.

Intra prediction of a current block may be performed using a referencesample derived based on a reconstructed sample included in a neighboringblock. Here, the reconstructed sample means that encoding/decoding iscompleted before encoding/decoding the current block. For example, intraprediction for the current block may be performed based on at least oneof reference samples P(−1, −1), P (−1, y) (0<=y<=2N−1) and P (x, −1)(0<=x<=2N−1). A prediction sample for the current block may be generatedby taking an average of reference samples, or copying a reference samplein a certain direction considering a directionality of an intraprediction mode of the current block.

Intra prediction of the current block may be performed using at leastone of a plurality of reference lines. All or a part of lengths of theplurality of reference lines may be determined as the same, or thelengths may be set different from each other.

For example, assuming that the current block has a W×H size, the k-threference line may include reference samples p(−k, −k), referencesamples on the same row as p(−k, −k) (e.g., reference samples fromp(−k+1, −k) to p(W+H+2(k−1), −k) or reference samples from p(−k+1, −k)to p(2W+2(k−1), −k)), and reference samples on the same column as p(−k,−k) (e.g., reference samples from p(−k, −k+1) to p(−k,W+H+2(k−1)) orreference samples from p(−k, −k+1) to p(−k,2H+2(k−1))).

FIG. 15 is a diagram exemplifying a plurality of reference sample lines.As in the example shown in FIG. 15 , when the first reference lineadjacent to the boundary of the current block is referred to as a‘reference line 0’, the k-th reference line may be configured to beadjacent to the (k−1)-th reference line.

Alternatively, unlike that shown in FIG. 15 , it may be configured thatall the reference lines have the same number of reference samples.

The intra prediction of the current block may be performed based onreference samples included in at least one reference line selected fromthe plurality of reference lines. Here, performing an intra predictionby selecting at least one of the plurality of reference line candidatesmay be referred to as an ‘intra prediction method using extendedreference sample (extended reference intra prediction)’ or an ‘extendedintra prediction method’. Further, the plurality of reference lines maybe referred to as an ‘extended reference lines’.

Whether to perform intra prediction based on the extended reference linemay be determined based on information signaled through a bitstream.Here, the information may be a 1-bit flag, but is not limited thereto.Information on whether to perform intra prediction based on the extendedreference line may be signaled in a unit of coding tree unit, codingunit, or prediction unit, or may be signaled in a unit of a sequence, apicture, or a slice. That is, whether to perform intra prediction basedon the extended reference line may be determined in a unit of asequence, a picture, a slice, a CTU, a CU, or a PU.

Since intra prediction generates prediction samples using limitedreference samples, the generated prediction samples may not reflectfeatures of the original image. That is, since the intra prediction ofthe current block is performed using only the neighboring samplesadjacent to the current block, the features of the original image maynot be accurately reflected. For example, in a case where an edge existsin the current block, a new object appears around a boundary of thecurrent block, or the like, the difference between prediction sample andthe original image may be large, depending on a position of a predictionsample in the current block.

In this case, the residual value becomes relatively large, and theamount of bits to be encoded/decoded may increase. Accordingly, thepresent invention proposes a method of refinement of a prediction samplegenerated through intra prediction. A method for modifying a predictionsample will be described in detail with reference to FIGS. 16 to 24 .

FIG. 16 is a flowchart illustrating a method of modifying a predictionsample according to an embodiment of the present invention.

First, an intra prediction image (a prediction block or a predictionsample) for the current block may be obtained, based on the intraprediction mode of the current block S1610. In the followingembodiments, a prediction image generated based on an intra predictionmode for a current block is referred to as a first intra predictionimage or a first prediction block, and a sample included in the firstintra prediction image is referred to as a first prediction sample.

When a first intra prediction image is generated as a result ofperforming intra prediction, an offset is applied to at least a part ofa current block to generate a second intra prediction image (or a secondprediction sample) refined from a first intra prediction image (or afirst prediction sample) may be generated. For example, refinement ofthe first intra prediction image based on the offset may be performed onan entire region of the current block, a pre-defined partial region, ora sample at a specific position. An offset used for refining the firstintra prediction image to the second intra prediction image may bereferred to as an ‘intra refinement offset’.

Whether to apply an offset to at least a partial region of the currentblock may be determined based on a type of the intra prediction mode ofthe current block, a directionality of the intra prediction mode, anangle of the intra prediction mode, a size or a shape of the currentblock (or the prediction block), or the like. For example, it may beconfigured that, when the intra prediction mode of the current block isa non-directional mode such as a DC mode or a planar mode, an offset isapplied to at least a partial region of the current block, whereas whenthe intra prediction mode of the current block is a directional mode, anoffset is not applied to the current block. As another example, it maybe configured that, when the intra prediction mode of the current blockis an intra prediction mode in a specific direction, an offset isapplied to at least a partial region of the current block.

Alternatively, whether to apply an offset to at least a partial regionof the current block may be determined by information decoded from abitstream. For example, a syntax ‘is_predblock_refinement_flag’indicating whether to refine the first intra prediction image using anoffset may be signaled through a bitstream. When a value of‘is_predblock_refinement_flag’ is 1, refinement of the first intraprediction image may be performed using the offset in the current block,whereas when the value of ‘is_predblock_refinement_flag’ is 0,refinement of the first intra prediction image may not be performed inthe current block. When the refinement of the first intra predictionimage is not performed, the first intra prediction image may be outputas a final prediction result of the current block.

To apply an offset to at least a partial region of the current block, anoffset for the current block may be determined S1620.

The offset may be defined for each predetermined unit in the currentblock. Here, the predetermined unit may mean one sample, a line (e.g.,row or column) including a plurality of samples, a sub-block including aplurality of lines, a sub-block of a predetermined size, or the like.For example, after defining an offset for each sample in the first intraprediction image, a second intra prediction image may be generated byadding or subtracting an offset corresponding to each sample to or fromeach sample.

The offset may be derived from a reference sample of the current block.Specifically, the offset may be derived based on a sum, a difference, anaverage, a weighted operation value (e.g., a weighted average), anintermediate value, a maximum value, a minimum value, or the like, of aplurality of reference samples (e.g., two, three, or more referencesamples).

Reference samples used for deriving the offset may include at least oneof a reference sample at a fixed position or a reference sampledetermined dependently on a position of a prediction sample (i.e., asample of the first intra prediction image). For example, the offset maybe derived based on at least one of a reference sample adjacent to thetop left corner of the current block and a reference sample on the samevertical line/horizontal line as the prediction sample to which theoffset is to be applied.

Alternatively, the number and/or position of the reference samples usedfor calculating the offset may be variably determined based on a size, ashape, a direction or an angle of the intra prediction mode of thecurrent block, or the like.

The offset may be derived using a reference sample before a filter(e.g., an AIS filter) is applied thereto. That is, the first intraprediction image may be generated using a reference sample to whichfiltering has been performed, while the second intra prediction imagemay be generated using a reference sample before a filter is appliedthereto.

The offset may be derived based on a plurality of reference samplesincluded in the same reference line, or may be derived based onreference samples each included in a different reference line.

For example, in the example shown in FIG. 15 , the offset may bedetermined based on a difference value between a value of a referencesample derived from the first reference line and a value of a referencesample derived from one of the second reference line or the fourthreference line.

Equations 7 and 8 show an example of deriving an intra refinementoffset. The intra refinement offset h may be determined using one of theequations shown in Equation 7.h=ref1(i ₁ ,j ₁)−ref0(i ₀ ,j ₀)h=ref2(i ₂ ,j ₂)−ref0(i ₀ ,j ₀)h=ref3(i ₃ ,j ₃)−ref0(i ₀ ,j ₀)h=ref4(i ₄ ,j ₄)−ref0(i ₀ ,j ₀)

Alternatively, a plurality of intra refinement offsets f0, f1, f2, andf3 may be determined as shown in Equation 8. The refinement of thecurrent block may be performed based on any one of the plurality ofintra refinement offsets f0 to f3, or may be performed by applying adifferent intra refinement offset in a unit of a predetermined region.f0=w₀*(ref1(i ₁ ,j ₁)−ref0(i ₀ ,j ₀))f1=w₁*(ref2(i ₂ ,j ₂)−ref0(i ₀ ,j ₀))f0=w₂*(ref3(i ₃ ,j ₃)−ref0(i ₀ ,j ₀))f0=w₃*(ref4(i ₄ ,j ₄)−ref0(i ₀ ,j ₀))  [Equation 8]

In Equations 7 and 8, refn (x=0 to 4) indicates the reference line n. Inaddition, (i_(n), j_(n)) represents a position of the reference sampleincluded in the reference line n. The value of the reference sample usedfor deriving the offset may be derived from neighboring reconstructedsamples for performing an intra prediction of the current block, or mayrepresent a separate reconstructed sample for calculating the offset.Here, the value of the separate reconstructed sample may be derived froma neighboring sample of the separate reconstructed sample or a sample ata predefined position. The value of the reference sample used forderiving the offset may be a value before an in-loop filter is appliedto a neighboring block or a value after the in-loop filter has beenapplied to the neighboring block.

The offset may be obtained by applying weights to the reference samples.For example, in Equation 8, an offset is obtained by applying a weight wto a differential value between reference samples. In addition, anoffset may be obtained through a weighted operation that applies adifferent weight to each reference sample. Here, a weight applied toeach reference sample may be determined based on a distance between asample at a specific position in the current block and a reference line,a distance between a sample at a specific position and a referencesample, a distance between reference samples, a distance betweenreference lines, or the like. The sample at the specific position mayrepresent a sample at a predefined position in the current block, or mayrepresent a sample to which an offset is to be applied (hereinafter,referred to as ‘an offset application target sample’, or ‘a refinementtarget sample’). The sample at the predefined position may include atleast one of a sample adjacent to the left boundary of the currentblock, a sample adjacent to the top boundary of the current block, or asample adjacent to the top left corner of the current block.

For example, a weight may be determined in proportion to a distancebetween a sample at a specific position in the current block and areference sample used for calculating an offset. For example, it isassumed that an offset is derived based on a reference sample at the topleft corner of the current block, a reference sample in a verticaldirection with respect to the offset application target sample, and areference sample in a horizontal direction with respect to the offsetapplication target sample. In this case, a weight applied to thereference sample at the top left corner of the current block may beobtained using at least one of an x-axis distance differential value ora y-axis distance differential value between the corresponding referencesample and the offset application target sample. For the sample in thevertical direction with respect to the offset application target sample,it may be obtained based on a y-axis distance differential value betweenthe corresponding reference sample and the offset application targetsample, and for the sample in the horizontal direction with respect tothe offset application target sample, it may be obtained based on thex-axis distance differential value between the corresponding referencesample and the offset application target sample.

Alternatively, a weight may be determined by adding/subtracting adistance between a sample at a specific position in the current blockand a reference sample used for calculating an offset to/from apredefined value.

As another example, a weight may be determined based on a ratio betweena value indicating a distance between a sample at a specific position inthe current block and a reference sample and a value indicating a sizeof the current block.

An intra refinement offset may be derived based on a residual sample ofa neighboring reconstructed sample (i.e., a reference sample) adjacentto the first intra prediction image. Here, the position of theneighboring reconstructed sample may be specified by at least one of atype, a direction, or an angle of the intra prediction mode used forobtaining the first intra prediction image. For example, the position ofthe neighboring reconstructed sample may be that of a reference samplepositioned on the angular line of the directional intra prediction modeor may be on a line that is orthogonal to the angular line of the intraprediction mode.

Alternatively, the offset value may be signaled in a unit of a slice, acoding unit, or a prediction unit.

When the offset is determined, an offset may be applied to the firstintra prediction image to generate a second intra prediction image whichis a refinement of the first intra prediction image S1630. For example,the following Equation 9 shows an example of deriving a sample P′ (x, y)of a second intra prediction image from a sample P(x, y) included in afirst intra prediction image.P′(i,j)=P(i,j)+j  [Equation 9]

As shown in Equation 8, the sample P′ (i, j) of the second intraprediction image may be obtained by adding the offset f to the sampleP(i, j) of the first intra prediction image. In contrast to Equation 9,the second intra prediction image may be obtained by subtracting theoffset from the first intra prediction image. Alternatively, the secondintra prediction image may be obtained based on a weighted operation ofthe first intra prediction image and the offset. Here, the weightsapplied to the first intra prediction image and the offset may bedetermined according to the position of the sample to be refined, or thelike.

In the case of a directional intra prediction mode, since a predictionsample is generated by copying a reference sample adjacent to thecurrent block, prediction efficiency in a region far from the currentblock may be deteriorated. That is, when the directional intraprediction mode is used, a residual value in a region relatively farfrom the boundary of the current block may include a large amount ofhigh-frequency components, which may result in a deterioration inencoding/decoding efficiency.

To solve the above problem, a method for refining a prediction image ina unit of a sub-block may be considered. Here, refining a predictionimage in a unit of a sub-block may mean that performing a refinement ofa prediction image only in a region corresponding to a predeterminedsub-block among an entire region of the current block, or an offset isdefined in units of a predetermined sub-block in the current block(i.e., using a different offset for each sub-block). When the predictionimage is refined in a unit of a sub-block, the accuracy of prediction ina region relatively far away from the block boundary may be improved.Hereinafter, referring to FIG. 17 , a method of refining a predictionimage in a unit of a sub-block will be described in detail.

FIG. 17 is a flowchart illustrating a method for refining a predictionimage in a unit of a sub-block according to an embodiment of the presentinvention.

Referring to FIG. 17 , for the current block, it may be determinedwhether to refine (or update) the first intra prediction image in a unitof a sub-block S1700. Whether the refinement of the first intraprediction image is performed in a unit of a sub-block may be determinedbased on at least one of a size, a shape of the current block, a type, adirection, or an angle of the intra prediction mode. Alternatively,whether to refine the first intra prediction image in a unit of asub-block may be determined by a flag decoded from a bitstream. Forexample, a syntax ‘is_sub_block_refinement_flag’ indicating whether toupdate the first intra prediction image in a unit of a sub-block may besignaled through a bitstream. When a value of‘is_sub_block_refinement_flag’ is 1, refinement of the first predictionsample using an offset may be performed in a predetermined sub-block inthe current block, whereas when the value of‘is_sub_block_refinement_flag’ is 0, the refinement in units of asub-block may not be performed.

Whether to refine the first intra prediction image and the refinementunit may be hierarchically determined. For example, only when it isdetermined that the refinement of the first intra prediction image isperformed, it may be determined whether the refinement unit is asub-block unit.

When it is determined that the refinement of the first intra predictionimage is performed in a unit of a sub-block, the intra predictionpattern of the current block may be determined S1710. Through the intraprediction pattern, it may be determined that all or a partial region ofthe current block to which the offset is applied, the partition type ofthe current block, whether the offset is applied to the sub-blockincluded in the current block, a size/sign of the offset assigned toeach sub-block, or the like.

Any one of a plurality of patterns predefined in the encoder/decoder maybe selectively used for an intra prediction pattern of the currentblock, and for that, an index specifying the intra prediction pattern ofthe current block may be signaled from a bitstream. As another example,the intra prediction pattern of the current block may be determinedbased on a partition mode of the prediction unit or coding unit of thecurrent block, a block size/shape, whether being a directional intraprediction mode, an angle of the directional intra prediction mode, anintra prediction pattern of a neighboring block, or the like.

Whether an index indicating an intra prediction pattern of the currentblock is signaled may be determined by predetermined flag informationsignaled from a bitstream. For example, when the flag informationindicates that the index indicating the intra prediction pattern of thecurrent block is signaled from the bitstream, the intra predictionpattern of the current block may be determined based on the indexdecoded from the bitstream. Here, the flag information may be signaledin a unit of at least one of a picture, a slice, or a block level.

When the flag information indicates that the index indicating the intraprediction pattern of the current block is not signaled from thebitstream, the intra prediction pattern of the current block may bedetermined based on a partition mode of the prediction unit or codingunit of the current block, or the like. For example, a partition type inwhich the current block is partitioned into sub-blocks may have the sametype as a partition type in which the coding block is partitioned intoprediction units.

When the intra prediction pattern of the current block is determined, anoffset may be obtained in a unit of a sub-block S1720. The offset may besignaled in a unit of a slice, a coding unit, or a prediction unit. Asanother example, the offset may be derived from a neighboring sample(e.g., a reference sample) of the current block. The offset may includeat least one of offset size information or offset sign information.Here, the offset size information may be within a range of an integergreater than or equal to zero.

When the offset is determined, for each sub-block, the second intraprediction image may be generated by refining the first intra predictionimage S1730. The second intra prediction image may be obtained byapplying an offset to the first intra prediction image. For example, thesecond prediction sample may be obtained by adding or subtracting theoffset to or from the first prediction sample. Here, a different offsetfor each sub-lock may be applied to the first intra prediction image.

FIGS. 18 to 22 are diagrams illustrating intra prediction patterns of acurrent block according to an embodiment to which the present inventionis applied.

For example, in the example shown in FIG. 18 , when the index is ‘0’ or‘1’, the current block is partitioned into top and bottom sub-blocks, anoffset may not be set for the top sub-block, and an offset ‘f’ may beset for the bottom sub-block. Accordingly, the first prediction sample(P(i,j)) may be used as it is for the top sub-block, and the secondprediction sample (P(i,j)+f or P(i,j)−f) generated by adding orsubtracting the offset to or from the first prediction sample may beused for the bottom sub-block. In the present specification, ‘not set’may mean that no offset is assigned to the corresponding block, or anoffset of ‘0’ is assigned to the corresponding block.

When the index is ‘2’ or ‘3’, the current block is partitioned into leftand right sub-blocks, an offset may not be set for the left sub-block,and an offset ‘f’ is set for the right sub-block. Accordingly, the firstprediction sample (P(i,j)) may be used as it is for the left sub-block,and the second prediction sample (P(i,j)+f or P(i,j)−f) generated byadding or subtracting the offset to or from the first prediction samplemay be used for the right sub-block.

Available intra prediction patterns may have limited in its range basedon the intra prediction mode of the current block. For example, when theintra prediction mode of the current block is a vertical directionalintra prediction mode or a prediction mode having a similar direction tothe vertical directional intra prediction mode (for example, when theintra prediction mode index is 22 to 30 among 33 directional predictionmodes), only an intra prediction pattern (e.g., index 0 or index 1 inFIG. 18 ) horizontally partitioning the current block may be applied tothe current block.

As another example, when the intra prediction mode of the current blockis a horizontal directional intra prediction mode or a prediction modehaving a similar direction to the horizontal directional intraprediction mode (for example, when the intra prediction mode index is 6to 14 among 33 directional prediction modes), only an intra predictionpattern (e.g., index 2 or index 3 in FIG. 18 ) vertically partitioningthe current block may be applied to the current block.

Alternatively, an intra prediction pattern available to the currentblock may be determined depending on whether the intra prediction modeof the current block is a non-directional mode.

FIG. 18 shows an example that no offset is set for one of sub-blocksincluded in the current block, and an offset is set for the other.Whether to set an offset for a sub-block may be determined based oninformation signaled for each sub-block.

Alternatively, whether to set an offset for a sub-block may bedetermined based on a position of the sub-block, an index foridentifying the sub-block in the current block, or the like. Forexample, with respect to a predetermined boundary of the current block,an offset may not be set for a sub-block adjoining the predeterminedboundary, and an offset may be set for a sub-block not adjoining thepredetermined boundary. Here, the predetermined boundary may bedetermined based on a size, a shape, or an intra prediction mode of thecurrent block.

When the predetermined boundary is assumed to be a top boundary of thecurrent block, for the intra prediction pattern corresponding to theindex ‘0’ or ‘1’, an offset may not be set for a sub-block adjoining thetop boundary of the current block, and an offset may be set for asub-block not adjoining the top boundary of the current block.

When the predetermined boundary is assumed to be a left boundary of thecurrent block, for the intra prediction pattern corresponding to theindex ‘2’ or ‘3’, an offset may not be set for a sub-block adjoining theleft boundary of the current block, and an offset may be set for asub-block not adjoining the left boundary of the current block.

In FIG. 18 , it is assumed that no offset is set for one of thesub-blocks included in the current block, and an offset is set for theother. As another example, an offset of a different value may be set foreach of the sub-blocks included in the current block.

Referring to FIG. 19 , an example in which a different offset is set foreach sub-block will be described.

Referring to FIG. 19 , when the index is ‘0’ or ‘1’, an offset ‘h’ maybe set for the top sub-block in the current block, and an offset ‘f’ maybe set for the bottom sub-block in the current block. Accordingly, asecond prediction sample (P(i,j)+h or P(i,j)−h) obtained by adding orsubtracting the offset ‘h’ to or from the first prediction sample may begenerated for the top sub-block, and a second prediction sample(P(i,j)+f or P(i,j)−f) obtained by adding or subtracting the offset ‘f’to or from the first prediction sample may be generated for the bottomsub-block.

Referring to FIG. 19 , when the index is ‘2’ or ‘3’, an offset ‘h’ maybe set for the left sub-block in the current block, and an offset ‘f’may be set for the right sub-block in the current block. Accordingly, asecond prediction sample (P(i,j)+h or P(i,j)−h) obtained by adding orsubtracting the offset ‘h’ to or from the first prediction sample may begenerated for the left sub-block, and a second prediction sample(P(i,j)+f or P(i,j)−f) obtained by adding or subtracting the offset ‘f’to or from the first prediction sample may be generated for the rightsub-block.

In FIGS. 18 and 19 , it is described that the current block ispartitioned into two sub-blocks having the same size, but the number ofsub-blocks included in the current block and/or the size of thesub-block is not limited to examples shown in FIGS. 18 and 19 . Thenumber of sub-blocks included in the current block may be three or more,and each sub-block may have a different size.

When a plurality of intra prediction patterns are available, theavailable intra prediction patterns may be grouped into a plurality ofcategories. In this case, the intra prediction pattern of the currentblock may be selected based on a first index for identifying a categoryand a second index for identifying an intra prediction pattern in thecorresponding category.

Referring to FIG. 20 , an example in which an intra prediction patternof a current block is determined based on a first index and a secondindex will be described.

In the example shown in FIG. 20 , twelve intra prediction patterns maybe classified into three categories each including four intra predictionpatterns. For example, the intra prediction patterns corresponding tothe indices 0 to 3 are classified into category 0, the intra predictionpatterns corresponding to the indices 4 to 7 are classified intocategory 1, and the intra prediction patterns corresponding to theindices 8 to 11 are classified into category 2.

A decoder may decode the first index from a bitstream to specify acategory including at least one intra prediction pattern. In the exampleshown in FIG. 20 , the first index may specify any one of categories 0,1, and 2.

When the category is specified based on the first index, the intraprediction pattern of the current block may be determined based on thesecond index decoded from the bitstream. When category 1 is specified bythe first index, the second index may specify any one of the four intraprediction patterns (i.e., index 4 to index 7) included in category 1.

In FIG. 20 , it is shown that each category includes the same number ofintra prediction patterns, but each category does not necessarily haveto include the same number of intra prediction patterns.

The number of available intra prediction patterns or the number ofcategories may be determined in a unit of a sequence or a slice. Inaddition, at least one of the number of available intra predictionpatterns or the number of categories may be signaled through a sequenceheader or a slice header.

As another example, the number of available intra prediction patternsand/or the number of categories may be determined based on a size of aprediction unit or a coding unit of the current block. For example, whenthe size of the current block (e.g., the coding unit of the currentblock) is 64×64 or more, the intra prediction pattern of the currentblock may be selected from the six intra prediction patterns shown inFIG. 21 . Alternatively, when the size of the current block (e.g., thecoding unit of the current block) is smaller than 64×64, the intraprediction pattern of the current block may be selected from the intraprediction patterns shown in FIG. 18 , FIG. 19 , or FIG. 20 .

In FIGS. 18 to 21 , it is shown that the sub-blocks included in eachintra-prediction pattern are rectangular. As another example, intraprediction patterns in which at least one of the sizes or shapes ofsub-blocks are different may be used. For example, FIG. 22 shows anexample of an intra prediction pattern having different sizes and shapesof sub-blocks.

An offset for each sub-block (e.g., offset h, f, g, or i for eachsub-block shown in FIG. 18 to FIG. 22 ) may be decoded from a bitstream,or may be derived from a neighboring sample (e.g., a reference sample)adjacent to the current block. For example, as described with referenceto FIG. 16 , an offset derived from at least one reference sample may beused for a sub-block, or an offset derived from reference samplesincluded in different reference lines (see Equations 7 and 8) may beused for a sub-block.

As another example, the offset of the sub-block may be determined inconsideration of a distance from a sample at a specific position in thecurrent block. For example, the offset may be determined in proportionto a value indicating a distance between a sample at a predeterminedposition in the current block and a sample at a predetermined positionin the sub-block.

As another example, the offset of the sub-block may be determined byadding or subtracting a value determined based on a distance between asample at a predetermined position in the current block and a sample ata predetermined position in the sub-block to or from a predeterminedvalue.

As another example, the offset may be determined based on a ratiobetween a value indicating a distance between a sample at apredetermined position in the current block and a sample at apredetermined position in the sub-block, and a value indicating a sizeof the current block.

Here, the sample at a predetermined position in the current block mayinclude a sample adjacent to the left boundary of the current block, asample located at the top boundary of the current block, a sampleadjacent to the top left corner of the current block, or the like.

In a sub-block, a different offset may be applied in a predeterminedunit. Specifically, a different offset may be applied for each sample inthe sub-block, or a different offset may be applied for each line (rowor column) or for each plurality of lines.

FIGS. 23 and 24 illustrate examples in which a different offset isapplied in a predetermined unit in a sub-block.

The left diagram of FIG. 23 shows a case in which a different offset isapplied in units of a row in a sub-block, and the right diagram shows anexample in which a different offset is applied in units of a column inthe sub-block. As shown in FIG. 23 , a different offset may be used foreach line, such as offset f0 in the first line, offset f1 in the secondline, offset f2 in the third line, and offset f3 in the fourth line.Here, f0 through f3 may be derived based on a differential value betweenthe reference samples each belonging to a different reference line, asdescribed with reference to Equation 8.

Alternatively, as shown in FIG. 24 , the weights to be applied to theoffsets may be set differently according to the positions in thesub-block. For example, a different offset may be used for each line,such as offset h in the first line, offset 2h in the second line, offset3h in the third line, and offset 4h in the fourth line. Here, h may bederived based on a difference value between the reference samples eachbelonging to a different reference line, as described with reference toEquation 7.

FIG. 25 is a flowchart illustrating processes of obtaining a residualsample according to an embodiment to which the present invention isapplied.

First, a residual coefficient of a current block may be obtained S2510.A decoder may obtain a residual coefficient through a coefficientscanning method. For example, the decoder may perform a coefficient scanusing a diagonal scan, a jig-zag scan, an up-right scan, a verticalscan, or a horizontal scan, and may obtain residual coefficients in aform of a two-dimensional block.

An inverse quantization may be performed on the residual coefficient ofthe current block S2520.

It is possible to determine whether to skip an inverse transform on thedequantized residual coefficient of the current block S2530.Specifically, the decoder may determine whether to skip the inversetransform on at least one of a horizontal direction or a verticaldirection of the current block. When it is determined to apply theinverse transform on at least one of the horizontal direction or thevertical direction of the current block, a residual sample of thecurrent block may be obtained by inverse transforming the dequantizedresidual coefficient of the current block S2540. Here, the inversetransform can be performed using at least one of DCT, DST, and KLT.

When the inverse transform is skipped in both the horizontal directionand the vertical direction of the current block, inverse transform isnot performed in the horizontal direction and the vertical direction ofthe current block. In this case, the residual sample of the currentblock may be obtained by scaling the dequantized residual coefficientwith a predetermined value S2550.

Skipping the inverse transform on the horizontal direction means thatthe inverse transform is not performed on the horizontal direction butthe inverse transform is performed on the vertical direction. At thistime, scaling may be performed in the horizontal direction.

Skipping the inverse transform on the vertical direction means that theinverse transform is not performed on the vertical direction but theinverse transform is performed on the horizontal direction. At thistime, scaling may be performed in the vertical direction.

It may be determined whether or not an inverse transform skip techniquemay be used for the current block depending on a partition type of thecurrent block. For example, if the current block is generated through abinary tree-based partitioning, the inverse transform skip scheme may berestricted for the current block. Accordingly, when the current block isgenerated through the binary tree-based partitioning, the residualsample of the current block may be obtained by inverse transforming thecurrent block. In addition, when the current block is generated throughbinary tree-based partitioning, encoding/decoding of informationindicating whether or not the inverse transform is skipped (e.g.,transform_skip_flag) may be omitted.

Alternatively, when the current block is generated through binarytree-based partitioning, it is possible to limit the inverse transformskip scheme to at least one of the horizontal direction or the verticaldirection. Here, the direction in which the inverse transform skipscheme is limited may be determined based on information decoded fromthe bitstream, or may be adaptively determined based on at least one ofa size of the current block, a shape of the current block, or an intraprediction mode of the current block.

For example, when the current block is a non-square block having a widthgreater than a height, the inverse transform skip scheme may be allowedonly in the vertical direction and restricted in the horizontaldirection. That is, when the current block is 2N×N, the inversetransform is performed in the horizontal direction of the current block,and the inverse transform may be selectively performed in the verticaldirection.

On the other hand, when the current block is a non-square block having aheight greater than a width, the inverse transform skip scheme may beallowed only in the horizontal direction and restricted in the verticaldirection. That is, when the current block is N×2N, the inversetransform is performed in the vertical direction of the current block,and the inverse transform may be selectively performed in the horizontaldirection.

In contrast to the above example, when the current block is a non-squareblock having a width greater than a height, the inverse transform skipscheme may be allowed only in the horizontal direction, and when thecurrent block is a non-square block having a height greater than awidth, the inverse transform skip scheme may be allowed only in thevertical direction.

Information indicating whether or not to skip the inverse transform withrespect to the horizontal direction or information indicating whether toskip the inverse transformation with respect to the vertical directionmay be signaled through a bitstream. For example, the informationindicating whether or not to skip the inverse transform on thehorizontal direction is a 1-bit flag, ‘hor_transform_skip_flag’, andinformation indicating whether to skip the inverse transform on thevertical direction is a 1-bit flag, ‘ver_transform_skip_flag’. Theencoder may encode at least one of ‘hor_transform_skip_flag’ or‘ver_transform_skip_flag’ according to the shape of the current block.Further, the decoder may determine whether or not the inverse transformon the horizontal direction or on the vertical direction is skipped byusing at least one of “hor_transform_skip_flag” or“ver_transform_skip_flag”.

It may be set to skip the inverse transform for any one direction of thecurrent block depending on a partition type of the current block. Forexample, if the current block is generated through a binary tree-basedpartitioning, the inverse transform on the horizontal direction orvertical direction may be skipped. That is, if the current block isgenerated by binary tree-based partitioning, it may be determined thatthe inverse transform for the current block is skipped on at least oneof a horizontal direction or a vertical direction withoutencoding/decoding information (e.g., transform_skip_flag,hor_transform_skip_flag, ver_transform_skip_flag) indicating whether ornot the inverse transform of the current block is skipped.

Although the above-described embodiments have been described on thebasis of a series of steps or flowcharts, they do not limit thetime-series order of the invention, and may be performed simultaneouslyor in different orders as necessary. Further, each of the components(for example, units, modules, etc.) constituting the block diagram inthe above-described embodiments may be implemented by a hardware deviceor software, and a plurality of components. Or a plurality of componentsmay be combined and implemented by a single hardware device or software.The above-described embodiments may be implemented in the form ofprogram instructions that may be executed through various computercomponents and recorded in a computer-readable recording medium. Thecomputer-readable recording medium may include one of or combination ofprogram commands, data files, data structures, and the like. Examples ofcomputer-readable media include magnetic media such as hard disks,floppy disks and magnetic tape, optical recording media such as CD-ROMsand DVDs, magneto-optical media such as floptical disks, media, andhardware devices specifically configured to store and execute programinstructions such as ROM, RAM, flash memory, and the like. The hardwaredevice may be configured to operate as one or more software modules forperforming the process according to the present invention, and viceversa.

INDUSTRIAL APPLICABILITY

The present invention may be applied to electronic devices which is ableto encode/decode a video.

The invention claimed is:
 1. A method of decoding an image with adecoding apparatus, comprising: determining, with the decodingapparatus, a candidate list of a current block in the image, thecandidate list including a plurality of candidates, the plurality ofcandidates being determined based on an intra prediction mode of aneighboring block adjacent to the current block; deriving, with thedecoding apparatus, an intra prediction mode of the current block basedon the candidate list and index information, the index informationspecifying one of the plurality of candidates in the candidate list;deriving, with the decoding apparatus, an extended intra prediction modebased on the intra prediction mode of the current block; and performing,with the decoding apparatus, intra-prediction on the current block basedon the extended intra prediction mode, wherein the intra prediction modeof the current block corresponds to one of pre-defined intra predictionmodes while the extended intra prediction mode does not belong to thepre-defined intra prediction modes, wherein the extended intraprediction mode has a different angle from directional prediction modesof the pre-defined intra prediction modes, and wherein whether to derivethe extended intra prediction mode is adaptively determined based on ashape of the current block.
 2. The method of claim 1, wherein inresponse to a case where the shape of the current block is one of squareand non-square, the extended intra prediction mode is derived from theintra prediction mode of the current block and the intra-prediction isperformed based on reference samples according to the extended intraprediction mode, and wherein in response to a case where the shape ofthe current block is the other of square and non-square, theintra-prediction is performed based on reference samples according tothe intra prediction mode of the current block without deriving theextended intra prediction mode from the intra prediction mode of thecurrent block.
 3. The method of claim 2, wherein the plurality ofcandidates includes an intra prediction mode having a value obtained byadding or subtracting a constant value to the intra prediction mode ofthe neighboring block, and wherein the constant value is greater than orequal to
 2. 4. The method of claim 3, wherein a maximum number of thecandidates included in the candidate list is equal to 3, 4, 5, or
 6. 5.A method of encoding an image with an encoding apparatus, comprising:determining, with the encoding apparatus, a candidate list of a currentblock in the image, the candidate list including a plurality ofcandidates, the plurality of candidates being determined based on anintra prediction mode of a neighboring block adjacent to the currentblock; determining, with the encoding apparatus, an intra predictionmode of the current block based on the candidate list, index informationbeing encoded to specify the intra prediction mode of the current blockamong the plurality of candidates in the candidate list; deriving, withthe encoding apparatus, an extended intra prediction mode based on theintra prediction mode of the current block; and obtaining, with theencoding apparatus, a residual block of the current block based on anoriginal block of the current block and a prediction block of thecurrent block, the prediction block being obtained based on the extendedintra prediction mode; and generating, with the encoding apparatus, abitstream by encoding the residual block, wherein the intra predictionmode of the current block corresponds to one of pre-defined intraprediction modes while the extended intra prediction mode does notbelong to the pre-defined intra prediction modes, wherein the extendedintra prediction mode has a different angle from directional predictionmodes of the pre-defined intra prediction modes, and wherein whether toderive the extended intra prediction mode is adaptively determined basedon a shape of the current block.
 6. A non-transitory computer-readablemedium for storing data associated with a video signal, comprising: adata stream stored in the non-transitory computer-readable medium, thedata stream comprising index information specifying one of a pluralityof candidates included in a candidate list, wherein the indexinformation is used to determine an intra prediction mode of the currentblock, wherein the plurality of candidates is determined based on anintra prediction mode of a neighboring block adjacent to the currentblock, and wherein an extended intra prediction mode is derived based onthe intra prediction mode of the current block and is used to performintra-prediction on the current block, wherein the intra prediction modeof the current block corresponds to one of pre-defined intra predictionmodes while the extended intra prediction mode does not belong to thepre-defined intra prediction modes, wherein the extended intraprediction mode has a different angle from directional prediction modesof the pre-defined intra prediction modes, and wherein whether to derivethe extended intra prediction mode is adaptively determined based on ashape of the current block.